This invention relates generally to analog to digital converters used for converting analog signals, such as audio signals, into digital signals, such as PCM signals. More particularly, the present invention is related to a technique for reducing noises which tend to occur during A/D conversion.
Recently, PCM recorders for recording PCM signals by means of a VTR or the like, and players for reproducing PCM disks in which audio signals have been recorded as PCM signals, have appeared in the market. In order to construct PCM devices for public use, it is needed to provide the same to consumers at low cost. However, there are many problems to be solved in providing PCM devices having a given performance by using low cost circuit parts.
One of the above-mentioned problems is to obtain an analog to digital converter (which will be referred to as an A/D converter hereafter) having given performance at low cost, which is used for converting analog signals into digital signals.
Many types of A/D converters are known, from an integration type to a high-speed parallel type, where an A/D converter of successive comparison or approximation type is usually used for A/D converting an acoustic signal (audio signal) in view of high-speed and accurate characteristics and the cost. Furthermore, since the input signal to be converted is an A.C. signal having positive and negative polarities, an A/D converter of offset binary type, which is D.C. biased at a point of one-half the full-scale range, is generally used.
However, in such a conventional A/D converter, error voltage is apt to occur in connection with most significant bit (MSB) applied to a D/A converter used in the A/D converter due to various reasons. Such an error voltage causes the MSB to assume an erroneous logic state, resulting in an false analog output of the D/A converter. This erroneous analog output is compared with the input analog signal so that undesirable noise signals will be superposed on the input analog signal. Especially, when the amplitude of the input analog signal is small, the superposed noises deteriorate the quality of the audio signal which will be reproduced by decoding the output digital signal from the A/D converter.
Although the above-mentioned problem of introduction of noise can be prevented if expensive parts having high accuracy are used for constructing an A/D converter, such a technique will raise the manufacturing cost, and thus is not suitable for A/D converters for public use.